Speedo reading wrong

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Cars Owned::Work in the real estate industry, but have outside interests in sports such as Rugby and jetboating. Spent 5 years restoring a 54 Citroen L15 that I still have, and 6 years restoring a 55 TR3, now I am onto a 62 TR4 restoring as a works replica look a like. possible the only one in NZ.

Happy new year, is there any "home hack" you can do to the speedo to get it more accurate. currently reads 80 mph when you are doing 60 mph on the gps, Need it reading more accurately for the new registration compliance. Or do I just have to get it recalibrated?

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The needle is only an interference fit on the spindle. It may have slipped round. It's not a major engineering operation to correct it, it needs care as the components are delicate. Just lift it off and replace in the right place!

james

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The needle is only an interference fit on the spindle. It may have slipped round. It's not a major engineering operation to correct it, it needs care as the components are delicate. Just lift it off and replace in the right place!

james

If it has slipped it may be possible to simple 'slip' it back rather than removing and refitting.

In my eyes there are two routes to go - depending on HOW the displayed speed is going wrong.
The position of the needle is defined by magnetic forces (rotating disc, driven by the speedo cable) and a little return spring (like a spiral).

If the deviation is a constant value (always showing 20 mph too much), this could be corrected by moving the needle 20 mph "back" - carefully,

If the deviation is a constant factor (always reading 25% too high) then the magnetism is too strong (less probable) or the return spring got weaker by time & age.
In this case the magnetism should be reduced or the spring strengthened (exchanged): Both a specialists job.

Regards, Johannes

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Cars Owned::Work in the real estate industry, but have outside interests in sports such as Rugby and jetboating. Spent 5 years restoring a 54 Citroen L15 that I still have, and 6 years restoring a 55 TR3, now I am onto a 62 TR4 restoring as a works replica look a like. possible the only one in NZ.

Have you checked the odometer against your GPS ? if you are clocking up the correct mileage, then the speedo is correct for your car, it just needs re-calibrating, this is done as mentioned above by adjusting the strength of the spinning magnet. Not much you can do to the hair spring, & that is unlikely to have changed.

If the miles covered is less than your odometer reads then you have the wrong speedo for your car. If it is a standard TR4 then the small number on the speedo dial (under the trip readout) should be close to 1180

Bob.

Edited January 26 by Lebro

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For most of my life of playing around with cars I could never figure out how to calibrate a speedo. I knew they worked by the magnetic coupling of a magnet and an ally disc, and so I (wrongly) assumed that it was all to do with the gap between the magnet and the disk. I bent a few in experiments before I realized that the answer must lie elsewhere.

Only in the past year, with the help of the internet, did I discovered the truth.
The following pages explain how to calibrate a speedo for both over and under reading.
It all seems obvious in principle. But I wonder how many people actually do it themselves in practice ???

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Cars Owned::Work in the real estate industry, but have outside interests in sports such as Rugby and jetboating. Spent 5 years restoring a 54 Citroen L15 that I still have, and 6 years restoring a 55 TR3, now I am onto a 62 TR4 restoring as a works replica look a like. possible the only one in NZ.

Just checked the number on the speedo and it is 1184. so it's correct for the TR4

so then I checked my TR3, it has always read wrong as well and it is number 1180. So next step is I will switch speedos and see if that reads any better and if so try that to see if that will get me through this speedo test that's part of registering a new vehicle on our roads.

Ian can you advise me what is the correct number range for a TR3 speedo..

Just checked the number on the speedo and it is 1184. so it's correct for the TR4

so then I checked my TR3, it has always read wrong as well and it is number 1180. So next step is I will switch speedos and see if that reads any better and if so try that to see if that will get me through this speedo test that's part of registering a new vehicle on our roads.

Ian can you advise me what is the correct number range for a TR3 speedo..

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I think we used to degauss things when I worked on the EMI-scanner, but it was a long time ago, and I’ve forgotten most of what I did there.

I expect I’ll spend a week playing around with magnets and bits of wire, get bored with it all and convert the speedo to electronic, with a pick up from the propshaft bolts. (This is why my TR is still a long way from being on the road. Always “Playing around”, not really “Getting on with it”.)

Happy new year, is there any "home hack" you can do to the speedo to get it more accurate. currently reads 80 mph when you are doing 60 mph on the gps, Need it reading more accurately for the new registration compliance. Or do I just have to get it recalibrated?

cheers jim

My 4A had the same problem. Had it serviced/recalibrated by JDO Instruments. Had to take a number of measurements for calibration. Detail is on their website. Speedo is now reading spot on against sat nav.

Alan.

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Early TRs might well have been sold with cross-ply tyres, and I think these would have been slightly larger than the radials which were fitted to most by the time TR4s were the current offering. This might account for the 1180/1184 figures.